Microwave heating of iron-rich Enteromorpha for Fe single-atom catalysts for the activation of peroxymonosulfate: Mechanisms and structure–activity relationships

To address the defects of uneven sites, atom aggregation, and poor pore structures of single-atom catalysts (SACs) prepared by conventional heating, this study prepared high-performance Fe SACs (Fe-NC) by microwave heating using Enteromorpha rich in iron and nitrogen as the precursor. The as-prepared catalysts were used to activate peroxymonosulfate (PMS) for the degradation of chloroquine phosphate (CQP). The results showed that hierarchical porous Fe-NCs with abundant surface functional groups, defective structures, and large specific surface area (SSA) were obtained by microwave heating by providing high-temperature hotspots. When using the catalyst (Fe-NC-800-15) prepared by microwave heating, the rates of PMS activation and CQP degradation were 74.48% and 99.76%, respectively, which were 8.84% and 55.65% higher than when using the catalyst prepared by conventional heating. Stronger signals of SO4·-, ·OH, 1O2, and ·O2–, lower interface resistance, and better electron transfer ability were the main reasons for better performance of the Fe-NC-800-15/PMS/CQP system. Quenching experiments showed that 1O2 and electron transfer were the dominant pathways for the degradation of CQP in the Fe-NC-800-15/PMS/CQP system. After 5 cycles of use, the degradation rate of CQP decreased to 63.37%. The deactivation of the catalyst was mainly caused by the slow circulation of Fe3+/Fe2+, loss of graphitic N, Fe leaching, reduction of defect degree and SSA, and consumption of OH/COOH and C≡N in the catalyst.